Aircraft often include steering systems to control the direction of movement of the aircraft when the aircraft is moving on the ground (“taxiing”). Typically, such steering systems include wheels forming part of a landing gear which is retracted into a landing gear bay after take-off and deployed from the landing gear bay prior to landing the aircraft. When on the ground, the aircraft is supported by the landing gear and moves by rotation of the landing gear wheels. In a common example, an aircraft includes a nose landing gear located under the aircraft nose and main landing gears located under the aircraft wings. The nose landing gear wheels are typically pivotable about a vertical axis to alter the direction of orientation of the nose landing gear wheels, thereby altering the direction of movement of the aircraft.
It is desirable that the steering system is correctly orientated (e.g. centered) when the aircraft lands, to correspond with the direction of travel of the aircraft at the point of landing. Deviation from the correct orientation may result in an uncomfortable or unsafe aircraft landing.
Typically, procedures for retraction of the landing gear include procedures for centering the nose landing gear prior to storing it in the landing gear bay. For example, sensor may detect that the a shock absorber on the landing gear is fully extended, indicating that the landing gear is not bearing the aircraft weight because the aircraft has left the ground, and automatically centre the landing gear wheels in response. However, the landing gear wheels may move away from the centre position due to, for example, equipment malfunction or accidental use of the steering control system during a flight, some aircraft include components to disable the steering system during the flight period, so as to prevent movement of the wheels. For example, some aircraft include a mechanically activated valve, known as a mechanical shut off valve, to isolate hydraulic actuators of the steering system from a source of hydraulic pressure. Such valves may be activated by a link arm or other structure which comes into contact with the valve when the landing gear is retracted. However, such valves add weight to the aircraft and increase costs.